1. Field of the Invention
This invention relates to a process and a machine for the wrapping and packaging of items in stretchable foils of soft plastic material.
2. Description of the Prior Art
It is known that the sale of many products of wide consumption is made markedly easier by suitably displaying and exhibiting the products themselves. On the other hand the manufacturers and retailers, both for the transportation and storage of the products and because of hygienic reasons, are compelled for the packaging of the articles before displaying and selling them.
With a view to reconciling the requirement of displaying the articles and the need to package them, it has been found useful, particularly in the foodstuff field, to provide a light package substantially consisting of a tray, box or similar container, made of cardboard, foamed polystyrene and the like, containing the goods to be packaged wrapped in a stretchable foil made of soft plastic material (such as modified polyvinylchloride), commonly known as "stretch film."
These foils show themselves particlarly useful for packaging foodstuff goods, even when such goods are liable to deteriorate, because they are air-permeable and chemically stable. It should however be noted that the foregoing stretchable foils also possess peculiar electrostatic characteristics which make them liable to rapid wear, do not present the least flexural strength, tend to get creased, thus taking up an unsightly aspect, and their resistance is markedly impaired by the tensile stresses to which they might have been subjected.
It can therefore be understood that the use of stretchable foils (whose thickness is generally 0.010 to 0.030 mm) for wrapping articles placed in containers or trays, involves several problems, whose solution is made more difficult by the ever increasing automation requirements.
In the prior art there are known various machines which are so devised as to carry out the packaging in stretchable foils of items, particularly of foodstuff goods, placed on carrying trays.
However, such machines carry out the wrapping of items in the stretchable foil without the foil being subjected to a previous complete stretching step. In fact, according to these prior art processes and machines, the stretching of the foil is brought about or completed upon and subsequently the foil being brought into contact with the items to be packaged. So far, for instance, in the U.S. Pat. No. 3,662,513 there is disclosed a process for the packaging of items, wherein a severed foil sheet of stretchable plastic material is placed under simple tension below a folding plate having a passage opening therethrough for the item to be packaged. The item is then raised vertically through the opening, while simultaneously holding fast at least two opposed edges of the foil sheet, which is thereby stretched while being drawn over the item to be packaged. Also according to the packing method disclosed in the Dutch patent application No. 7006535, published on Nov. 8, 1971, wherein the stretchable foil is subjected to stretch up to 20 percent in at least one direction prior to packing, the foil is caused to be further stretched subsequent to contact with the item to be packaged.
Consequently the stretchable foil is subjected, as a consequence of likely uneven contact with the item to be packaged, to differentiated stress, particularly when the items present, as a whole, an irregular upper surface, as in case of wrapping a number of fruits, such as apples, pears and the like, in a container.
These stresses bring about mechanical and electrical anisotropies, which are such as to likely cause the tearing and rapid wear of the foil that, in any case, wraps the goods in an uneven way. Besides, the machines of the prior art are extremely complex and consequently poorly reliable because of the various machine controls which are necessary with a view to making flexible the adjustment of the foil wrapping devices to the various dimensions of the goods and trays. Still another drawback presented by the known processes and machines is that stresses are brought about at peripheral zones of the foil during the stretching step thereof.
Lastly, a remarkable disadvantage presented by the known machines deals with the defective utilization of the stretchable foil which, still as a consequence of not being suitably pre-stretched, is consumed in amounts far larger than it would be required.